I suspect a large percentage of the AF QC complaints are actually related to the challenge of shooting wide open and nailing/keeping the plane of focus where you want. With a Canon L fast prime the gut reaction is "what am I doing wrong", but with a 3rd party fast prime it's "what did the manufacturer do wrong."
I'm sure that is not the only reason (but it might contribute to the Sigma's reputation). I own the Sigma myself, bought it with huge focus shift issues, sent it in for calibration, got it back in much better condition. But still the AF is not that reliable. Sometimes it just fucks up, sometimes it is slightly off in one direction, sometimes in the other.
I shot a Canon 1.4 for one afternoon and had a completely different experience. It was always spot on and I could trust the AF much more. Without any AFMA adjustments.
The Sigma is very useable after the calibration, but there is still a difference to the Canon. That said, I don't like the build quality of the Canon 1.4 and the Sigma has great IQ.